Residential Apartment Slab Design in Coastal Mumbai

Case Study 1: Residential Apartment Slab Design in Coastal Mumbai

Scenario

A 12-storey residential apartment complex is under construction in Bandra, Mumbai — a high-humidity, chloride-rich coastal environment. The ground-floor parking slab (one-way spanning) must support vehicle loads while minimizing deflection and corrosion risk. Key constraints include: limited site access for heavy reinforcement delivery, strict 40 mm minimum concrete cover (per IS 456:2000 for severe exposure), and a tight 8-week construction schedule requiring rapid formwork turnover.

Given Data

• Slab Thickness: 220 mm (increased from standard 200 mm to accommodate ducts and enhance durability)
• Span Length: 4.2 m (shorter span due to column grid optimization)
• Live Load: 7.5 kPa (accounting for SUV parking and occasional light commercial use)
• Concrete Strength: 30 MPa (M30 mix with slag cement for chloride resistance)
• Steel Yield Strength: 500 MPa (thermo-mechanically treated TMT bars, Fe500D grade)

Calculation

Using the Rebar Design Calculator:

1. Input values are entered directly: slab_thickness=220, span_length=4.2, live_load=7.5, concrete_strength=30, steel_yield_strength=500.
2. Internally, the tool applies ACI 318-19–informed simplified flexural design logic for one-way slabs:
   • Factored moment: $M_u = \frac{w_u L^2}{8}$, where $w_u = 1.5 \times \text{DL} + 1.5 \times \text{LL}$. Assuming self-weight DL ≈ 5.5 kPa (220 mm × 25 kN/m³), $w_u = 1.5(5.5 + 7.5) = 19.5\ \text{kPa}$ → $M_u = \frac{19.5 \times 4.2^2}{8} = 42.9\ \text{kN·m/m}$.
   • Required steel ratio: $\rho = \frac{0.85 f'_c}{f_y} \left[1 - \sqrt{1 - \frac{2 R_n}{0.85 f'_c}}\right]$, with $R_n = M_u/(0.9 \cdot d^2)$, $d = 220 - 40 = 180\ \text{mm}$. Solving yields $\rho \approx 0.0024$.
   • $A_s = \rho \cdot b \cdot d = 0.0024 \times 1000 \times 180 = 432\ \text{mm}^2/\text{m}$.
   • Spacing for 12 mm Ø bars ($A_{bar} = 113\ \text{mm}^2$): $s = \frac{1000 \cdot A_{bar}}{A_s} = \frac{1000 \times 113}{432} \approx 262\ \text{mm}$ → rounded down to 250 mm c/c for constructability and code compliance (IS 456 limits max spacing to $3d = 540\ \text{mm}$; 250 mm satisfies crack control).

Result and Decision

The calculator outputs:

• Rebar Area: 432.1 mm²/m
• Rebar Spacing: 261.7 mm → adopted as 250 mm c/c with 12 mm Ø Fe500D bars in top and bottom layers (dual-layer for shrinkage and temperature control per IS 456 Cl. 26.3.3). Final specification: 12 mm Ø @ 250 mm c/c, both ways (with 10 mm Ø @ 200 mm c/c for torsion at corners), 40 mm cover using corrosion-inhibiting admixture and epoxy-coated ties.

Lesson

In aggressive environments, increasing concrete strength and cover is necessary — but rebar spacing must be tightened (not widened) to maintain crack width control, even when calculated area appears conservative. Always verify spacing against serviceability limits, not just ultimate capacity.